Like most of the power supply products, DC/DC converters are developing towards high efficiency, high power density, high reliability and low cost. Since resonant converters usually employ a control method of Pulse Frequency Modulation (PFM), it is relatively easy to realize a zero voltage switching of switches therein and it is easy to meet the high efficiency and high power density requirements for converters. Thus, the resonant converters are widely used.
Although the resonant converters have advantages, such as high conversion efficiency, under a normal operation state, i.e., under a situation where a voltage gain is greater than or equal to a predetermined value, there are still some problems existing in the resonant converter which only employs frequency changing control when it operates under an abnormal operation state, e.g., when the voltage gain is less than the predetermined value (e.g., under a state where the circuit is started or an output current is limited or the like). When the resonant converter operates under a situation where the voltage gain is less than the predetermined value, the voltage gain of the circuit is required to be relatively low; especially, when the output current is limited, corresponding control needs to be applied on the current of the circuit to make the circuit be capable of operating normally. To meet these requirements, it is necessary to increase an operating frequency of the resonant converter. However, an increase of frequency means an increase of circuit loss. When the frequency is increased to a particular value, the entire loss of the converter will go beyond the capacity of the converter and then the converter will be damaged. FIG. 1 is a schematic diagram illustrating such a control method of only changing frequency, in which the abscissa represents a ratio fn of an operating frequency of a circuit to a resonant frequency of the resonant circuit, and the ordinate represents a phase-shifting angle θ.
The above control method of only changing frequency cannot resolve the problem of overlarge circuit loss which occurs when the voltage gain is less than the predetermined value. Another control method employs a method of both changing frequency and changing pulse width. As shown in FIG. 2, the abscissa represents the ratio fn of the operating frequency of the circuit to the resonant frequency of the resonant circuit, and the ordinate represents a duty cycle D of switches. The circuit follows an operation locus as below: firstly, from A to B, the circuit being controlled to operate through changing the duty cycle D of switches, and then entering into a BC stage in which the circuit is controlled to operate by adjusting the pulse width of the switches. Since the method of changing the pulse width is employed, the switching frequency is lower than that in the method of only changing frequency under the same gain. However, a hard-switching of switches at primary side may be brought out by reducing the pulse width, thereby resulting in that the entire loss is still large.